1. Field of the Invention
The present invention relates to an electro-photographic image forming apparatus such as a laser printer, a facsimile, a copier, etc., and more particularly, to an electro-photographic image forming apparatus preventing toner from scattering when printing.
2. Description of the Related Art
FIG. 1 shows schematically an example of an electro-photographic image forming apparatus using non-magnetic one-component toner, and FIG. 2 is a sectional view showing a developing part of the electro-photographic image forming apparatus in detail. Referring to FIGS. 1 and 2, a surface of a photosensitive medium 1 is charged by a charging roller 2, and a laser beam 4 emitted from a laser scanning unit 3 forms an electrostatic latent image thereon. The non-magnetic toner 8 is supplied to a developing roller 5 by a toner supply roller 6. The developing roller 5 is disposed opposite to the photosensitive medium 1 a predetermined distance apart from the photosensitive medium 1. The developing roller 5 rotates in the D2 direction shown in FIG. 2. The toner supply roller 6 is disposed inside a toner hopper 10 so as to be contacted with the developing roller 5. The toner supply roller 6 comprises a core shaft made of stainless steel, etc., and an elastic member such as urethane foam or silicone foam to cover the core shaft. The toner supply roller 6 rotates in the D3 direction in contact with the developing roller 5 so as to transport the non-magnetic toner 8 within the toner hopper 10 to the surface of the developing roller 5, and remove the toner, which remains on the surface of the developing roller 5 that is not used to develop the electrostatic latent image, from the surface of the developing roller 5. The toner is transported from the toner supply roller 6 to the developing roller 5 by an electrical field between the developing roller 5 and the toner supply roller 6. Here, reference number 22 is an electrical power source that applies the voltage to the toner supply roller 6. A regulating member 7 is disposed in contact with the surface of the developing roller 5 in a position next to the toner supply roller 6 and before a developing area A predetermined electric charge is applied to the toner transported to the developing roller 5 by high friction between the surface of the developing roller 5 and the regulating member 7 while the regulating member 7 regulates the toner at a predetermined thickness. The toner passed through the regulating member 7 develops the electrostatic latent image formed on the photosensitive medium 1. Because the developing roller 5 is at the predetermined gap apart from the photosensitive medium 1 and is applied with a square wave of an alternating current (AC) superposed with a direct current (DC) component by an electrical power source 21, the toner transported to the developing area by the developing roller 5 develops the electrostatic latent image on the surface of the photosensitive medium 1 while the toner reciprocates in the gap of the developing area. The toner developing the electrostatic latent image is transferred to a sheet of paper 13 by a transfer roller 11, and is fixed thereon by the fixing device 17. Thus, a printing operation is completed. Also, the toner, which remains on the surface of the photosensitive medium 1 after the electrostatic latent image is transferred to the sheet of paper 13, is removed by a cleaner 15 and collected into a used toner container 16.
In the case of non-contact development as described above, since the alternating current (AC) voltage superposed with the direct current (DC) is applied to the developing roller 5 when developing (an image area of the VDO-data chart of a control-timing diagram in FIG. 3), the toner blows and reciprocates in the gap of the developing area. Therefore, because toner that is insufficiently charged receives a weak electrical attraction force from the electric field formed in the gap of the developing area between the developing roller 5 and the photosensitive medium 1, the insufficiently charged toner is scattered inside the apparatus like the toner (T) shown in FIG. 4. Because airflow (F) caused by the rotation of the photosensitive medium 1 and the developing roller 5 is stronger as a printing speed increases, the toner scattered by the airflow is increased. Therefore, there are problems that the inside of the image forming apparatus is contaminated and a printing quality is deteriorated by loose toner contaminating images.